The invention relates to a compressor unit comprising a screw compressor or the like, and a drive motor which is connected for drive purposes to the compressor by way of an engageable and disengageable clutch and can be temporarily stopped in order to control the screw compressor throughput (intermittent control), during which the drive connection to the screw compressor is interrupted by the clutch.
In compressing air or other gases it is a known requirement to be able within a given operating time to vary the throughput of the compressor unit which performs the work of compression so that it corresponds to the actual compressed medium requirement. In the case of reciprocating compressors, various continuous adjustment methods are known, these mostly adapting the compressor throughput to the actual requirement to an exact degree by adjusting the working valves. In the case of screw compressors which have no control equipment comparable to the reciprocating compressor working valves, the throughput adjustment is done either by throttling the suction line or by temporarily stopping the compressor by switching off the drive motor (intermittent control). Control by throttling the suction line results in energy loss because the compressor and drive motor carry on running when there is no delivery. Disengaging the compressor also results in losses if the prime mover is not stopped at the same time.
Control methods for screw compressors are also known in which the drive motor is stopped together with the screw compressor as soon as the required pressure is reached in the distribution system supplied by the compressor or in a pressure vessel. In this case, however, there are problems in the restarting of the compressor unit when further delivery is required for supplying the system. This is because in the case of screw compressors when under stoppage there is no particular load torque active as such compressors are of the so-called internally non-tight type, in which the load torque is dependent on the speed of rotation. Nevertheless, the load torque rises rapidly with increasing speed of rotation, particularly if the screw compressor is to run against the full compression pressure, so that danger of thermal overloading of the drive motor exist, and in any event there is a comparatively long run-up time. With this control method, prompt resumption of delivery is not ensured. This drawback can only be mitigated by costly technology. Additional suction throttle control is therefore generally provided, with provision for pressure unloading of the screw compressor at standstill, so that a pressureless start is possible. This equipment and these provisions, to which the necessary controls must be added, result in considerable additional expense.
A further difficulty in the intermittent control of compressor units derives from the fact that the drive motors during startup consume a higher current than when they have attained their rated speed and full torque. Even when a screw compressor is started with pressure unloading, it can happen that if short operational pauses follow one after the other, i.e. during frequent starting within a given time period, the drive motor becomes thermally overloaded. Motor manufacturers therefore frequently prescribe how many starts per unit of time are allowed as a maximum, and this figure should not be exceeded.
If a clutch is used between the drive motor and screw compressor, costly control equipment for the engagement and disengagement of the clutch is required. On starting the compressor unit it must be checked that the clutch is disengaged, and on reaching a given speed of rotation the clutch must be engaged. On stopping the unit the clutch must be disengaged at the correct time. If the control equipment fails or operates inaccurately, e.g., because of incorrect setting, there is again danger of thermal overloading of the drive motor.